Center of rotation in simple tipping

 # When a simple tipping force is applied to the crown of an incisor, the center of rotation is usually located:
A. At the apex
B. At the incisal edge
C. At the cervical line
D. One third the root length from the apex

The correct answer is D. One third the root length from the apex.

Center of rotation-the point around which rotation actually occurs when an object is being moved. If a force and a couple are applied to an object, the center of rotation can be controlled and made to have any desired location. 

The center of resistance( Cp)f or any tooth is at the approximate midpoint of the embedded portion of the root. lf a single force is applied to the crown of a tooth, the tooth will not only translate but also rotate around Cp (i.e., the center of rotation and center of resistance are identical), because a moment is created by applying a forcea t a distance from Cn. The perpendiculadr istancef rom the point of force application to the center of resistance is the moment arm. Pressure in the periodontal ligament will be greatest at the alveolar crest and opposite the root apex. 

Ref: Contemporary Orthodontics, Proffit, 4th Edition, Page no. 373

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